Underground conduits and aerial cables which may, for example, carry multiple bundles of twisted wire pairs constituting telephone wiring, are normally pressurized with a dry gas to prevent ingress of moisture through, for example, leaky joints between conduit or cable sections. Moisture ingress may lead to condensation or build up. Unless prevented, the presence of moisture in the atmosphere inside these underground conduits and aerial cables may cause immediate and permanent damage to moisture sensitive materials therein or cause operation failures due to, for example, corrosion and voltage arcing.
Normally, the dry gas source supplying these underground conduits and aerial cables consists of one or more air dryers located at a central office. Standard air dryers nominally consist of air compressors, air drying devices, a dry air distribution network consisting of pneumatically coupled pipes and tubing, and associated operating controls. Cable dryers feed underground conduits, for example, through tubing that extends into underground vaults. These underground conduits must be pressurized along their entire length, which may extend sub-surface for many miles. Leaks and other losses create a variable demand on the supply of dry gas to maintain the desired pressure within the underground conduit. Moisture ingress is prevented by maintaining a constant positive pressure of dry gas on the underground conduit, such that the dry gas flows out of the underground conduit and blocks the inflow of moist ambient air.
Conventional gas dryers utilize established technologies for generating adequate amounts of dry gas to satisfy the variable demand. For example, many standard gas dryers use conventional “liquid-ring” style compressors, which are compact but are also complex and maintenance intensive. These compressors are forced to start and stop at intermittent intervals as the cyclic demand for dry gas fluctuates, which causes component wear that accelerates the maintenance requirement and may result in component failure.
Conventional gas dryers also rely on either conventional refrigerant drying or, more commonly, conventional pressure swing adsorption (PSA) to generate a stream of dry gas. PSA apparatus and the apparatus used to practice refrigerant drying are complex, maintenance intensive, and tend to be unreliable over extended operating periods. In particular, PSA technology uses a drying media which can become ineffective over time.
What is needed, therefore, is a gas dryer for underground conduits and aerial cables that has a simple construction and lasting reliability and methods of operating such gas dryers that overcome these and other disadvantages of conventional gas dryers and conventional methods of operating such gas dryers.